I think it's really a question of what we know and are used to.

If the entire storyline had been different, and a compelling novel and film had been written about Sonny's ascension as Don, and then 35 years later someone suggested that a story about Michael might've been more interesting, we'd be saying "Nah, what could be interesting about a story about a dopey, drippy college kid who wanted nothing to do with the family business."